1. Field of the Invention
The present invention relates to a fixing apparatus used in an image forming apparatus and the like.
2. Related Art
In an image forming apparatus and the like using an electrophotographic method that is widely applied in copying machines, laser printers, facsimiles, and the like, conventionally, a fixing method used in a fixing apparatus is generally a heat fixing method. For such a heat fixing method, a hot roller fixing method using a hot roller is generally applied.
With the hot roller fixing method, a hot roller, that includes an internal heater as a heat source and an external surface covered with rubber or resin having good releasability, and a pressure roller are pressed against each other so that a nip portion is formed between these rollers. Then, a transfer paper on which a toner image has been formed is caused to pass through the nip portion so that the toner is heated and melted. Thereby, the toner is fused and fixed onto the transfer paper. The hot roller fixing method is suitable for increasing speeds because the entire hot roller is maintained at a predetermined temperature.
In the fixing apparatus using the aforementioned hot roller and pressure roller, the pressure roller is pressed against the hot roller so that a fixing operation is reliably performed. Consequently, in the fixing apparatus using the hot roller and the pressure roller, a pressing means is generally used so that a pressure applied state in which the pressure roller is pressed against the hot roller can be formed.
The pressing means has a configuration in which, for example, bias is applied to the pressure roller via a spring or the like with respect to the hot roller, and the pressure roller is pressed against the hot roller so as to enter a pressure applied state.
In the image forming apparatus using the fixing apparatus including the aforementioned pressing means, when warming up or being on standby, operations in the image forming apparatus having been stopped, or the like, if the pressure roller is maintained for a long time in a state in which the pressure roller is pressing against the hot roller, the surface of the hot roller or the like may possibly be deformed.
Consequently, if the time period when the fixing apparatus does not perform a fixing operation is of a certain length, pressure applied by the pressure roller against the hot roller is released so that the high quality performance of the fixing apparatus can be maintained. Accordingly, in the fixing apparatus using the hot roller and the pressure roller, in order to switch between a pressure applied state in which the pressure roller is pressed against the hot roller and a pressure released state in which such pressure is released, a switching means capable of switching between the pressure applied state and the pressure released state with respect to the aforementioned pressing means is generally used (for example, see JP 2003-280308A).
The switching means switches between a pressure release operation and a pressure application operation as described above. The pressure release operation is an operation for shifting from the pressure applied state to the pressure released state with respect to the pressing means in the pressure applied state. On the contrary, the pressure application operation is an operation for shifting from the pressure released state to the pressure applied state with respect to the pressing means in the pressure released state.
Such a switching means is a means that, for example, includes a turnable cam with respect to the pressing means as described in the above example and switches to the pressure released state in which pressure applied by the pressure roller against the hot roller is reduced by turning the cam in a forward rotational direction with respect to the pressing means that is in the pressure applied state so that the pressure roller applies pressure to the pressing means in a direction opposite to the direction in which the bias is applied to the pressure roller so as to press the pressing means back. Also, the means switches to the pressure applied state by turning the cam in a backward rotational direction with respect to the pressing means that is in the pressure released state so as to release the pressure to the pressing means.
If the switching means including such a turnable cam as described above is used in the aforementioned fixing apparatus, the cam needs to be turned in order to perform switching. When the cam is turned, some problem or the like may occur to the mechanism of the cam so that the mechanism enters a locked state. Accordingly, there may be a case in which operations cannot be performed using the cam.
In order to forcibly turn the cam when such a state occurs, it is necessary to use a larger scaled motor or the like to turn the cam than necessary; thus, the cost of the fixing apparatus may possibly increase.
In general, as for a mechanism that turns in the forward rotational direction or the backward rotational direction, if the mechanism enters a locked state, it is a well known fact that the operations of the mechanism are often recovered by turning the mechanism once in a rotation direction opposite to the rotation direction before entering the locked state and, thereafter, turning again in the rotation direction before entering the locked state and the like.